As single-cell, eukaryotic microorganisms, yeasts can be cultured without difficulty and have the great advantage of being readily genetically manipulable. Also, they can process and modify recombinant proteins in accordance with the patterns known from higher organisms. As far as is yet known, they contain no pathogenic substances and are therefore also suitable for the production of therapeutic proteins. Thus for example the first vaccine produced by heterologous gene expression, the hepatitis B vaccine, was heterologously expressed in the well-characterised baker's yeast Saccharomyces cerevisiae (Lepetic et al., 1996).
While S. cerevisiae makes it possible to produce many different proteins (for a review, see Gellissen and Hollenberg, 1997), there are nonetheless also a few limiting properties. Thus for example the maximal content of heterologous protein is about 1–5% of the total protein content of the cell (Buckholz and Gleeson, 1991).
Various other yeasts, such as for example Schizosaccharomyces pombe, Kluyveromyces lactis, Yarrowia lipolytica and also Hansenula polymorpha have been characterised and compared with S. cerevisiae in terms of their suitability as expression systems (Müller et al., 1998). All these yeasts showed markedly stronger secretion of active protein than S. cerevisiae, the heterologous gene expression being dependent on the particular gene, but independent of the donor organism. Basically, methylotrophic yeasts are found to be attractive organisms for the production of recombinant proteins. The methylotrophic yeasts are subdivided into the four genera Hansenula, Pichia, Candida and Torulopsis, which all possess the ability to utilise methane, methylamine, formaldehyde or formate as carbon and energy source.
Inter alia, the yeast Hansenula polymorpha from the Saccharomycetaceae family (Lodder, 1970) belongs to the relatively small group of the methylotrophic yeasts. Hansenula polymorpha does not ferment glucose under aerobic conditions, and is thus Crabtree-negative (Verduyn et at., 1992) and with a growth temperature optimum of 37° C. is classed among the thermotolerant yeasts; thus it is an exception among the methylotrophic genera. The natural habitat of the methylotrophic yeasts is locations rich in organic material.
Hitherto, only a few genes of the yeast Hansenula polymorpha had been cloned and characterised (Hansen and Hollenberg, 1996), and according to GenBank the number of cloned genes in Hansenula polymorpha amounts to >30. However, how many of these are thoroughly characterised or are suitable as marker genes for plasmid selection, is unknown to me. Hence, I would suggest that similarities of these genes with the homologous genes in Saccharomyces cerevisiae, insofar as these are present, are relatively limited (Dobson et al., 1982). The key enzymes in methylotrophic metabolism are the enzymes methanol oxidase (MOX), dihydroxyacetone synthase (DAS) and formate dehydrogenase (FMD), whose controlled expression by very strongly regulated promoters opens up a variety of useful possibilities for heterologous gene expression. These facts make Hansenula polymorpha an industrially interesting organism (Gellissen et al., 1994).
So far, only two auxotrophic strains of H. polymorpha are available, whose transformation can be selected by functional complementation of ura3- or leu2-deficiency respectively. The provision of farther transformation systems, consisting of auxotrophic strains and of nucleic acids capable of complementing the auxotrophy hitherto failed because suitable genes capable of this complementation were not available. While a number of genes from the amino acid or nucleic acid biosynthesis pathway of the baker's yeast S. cerevisiae were known, it had nonetheless in the past been found that the differences between the genes of baker's yeast and the methylotrophic yeasts are sometimes considerable. Hence it cannot in general be expected that a S. cerevisiae gene will be suitable for complementation of an auxotrophy in a methylotrophic yeast, nor can it be expected that genes from a methylotrophic yeast will be capable of complementing an auxotrophy in S. cerevisiae. 
Hence a technical problem underlying the present invention is to provide a new gene from H. polymorpha, which can serve as a selectable marker for complementation in the transformation of suitable auxotrophic yeast strains. A further problem consists in the provision of vectors and host cells, which contain the gene. Further, it is to provide the polypeptide encoded by the gene and antibodies which specifically recognise the polypeptide. Finally, it is to provide suitable auxotrophic strains.